FIG. 1 shows an exemplary network structure of a Long-Term Evolution (LTE) system as a mobile communication system to which a related art and the present invention are applied. The LTE system is a system that has evolved from the existing UMTS system, and its standardization work is currently being performed by the 3GPP standards organization.
The LTE network can roughly be divided into an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) and a Core Network (CN). The E-UTRAN is generally comprised of a terminal (i.e., User Equipment (UE)), a base station (i.e., Evolved Node B (eNode B)), an access gateway (aGW) that is located at an end of the network and connects with one or more external networks. The access gateway may be divided into a part that handles processing of user traffic and a part that handles control traffic. In this case, the access gateway part that processes the user traffic and the access gateway part that processes the control traffic may communicate with a new interface. One or more cells may exist in a single eNB. An interface may be used for transmitting user traffic or control traffic between eNBs. The CN may include the aGW and a node or the like for user registration of the UE. An interface for discriminating the E-UTRAN and the CN may be used.
FIGS. 2 and 3 show respective exemplary structures of a radio interface protocol between the terminal and the E-UTRAN based on the 3GPP radio access network standards. The radio interface protocol has horizontal layers comprising a physical layer, a data link layer, and a network layer, and has vertical planes comprising a user plane (U-plane) for transmitting user data information and a control plane (C-plane) for transmitting control signaling. The protocol layers in FIGS. 2 and 3 can be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in the communication system. The radio protocol layers exist as pairs between the UE and the E-UTRAN and handle a data transmission in a radio interface.
The layers of the radio protocol control plane in FIG. 2 and those of the radio protocol user plane in FIG. 3 will be described as follows.
The physical layer, the first layer, provides an information transfer service to an upper layer by using a physical channel. The physical layer is connected to an upper layer called a medium access control (MAC) layer via a transport channel. Data is transferred between the MAC layer and the physical layer via the transport channel. The transport channel is divided into a dedicated transport channel and a common transport channel according to whether or not a channel is shared. Between different physical layers, namely, between a physical layer of a transmitting side and that of a receiving side, data is transmitted via the physical channel using radio resources.
The second layer includes various layers. First, a medium access control (MAC) layer performs mapping various logical channels to various transport channels and performs logical channel multiplexing by mapping several logical channels to a single transport channel. The MAC layer is connected to an upper layer called a radio link control (RLC) layer by a logical channel. The logical channel is roughly divided into a control channel that transmits information of the control plane and a traffic channel that transmits information of the user plane according to a type of transmitted information.
A Radio Link Control (RLC) layer of the second layer segments and/or concatenates data received from an upper layer to adjust the data size so as for a lower layer to suitably transmit the data to a radio interface. In addition, in order to guarantee various Quality of Services (QoSs) required by each radio bearer (RB), the RLC layer provides three operational modes: a Transparent Mode (TM); an Unacknowledged Mode (UM); and an Acknowledged Mode (AM). In particular, the AM RLC performs a re-transmission function through an Automatic Repeat and Request (ARQ) for a reliable data transmission.
A Packet Data Convergence Protocol (PDCP) layer of the second layer performs a function called header compression that reduces the size of a header of an IP packet, which is relatively large and includes unnecessary control information, in order to effectively transmit the IP packet such as an IPv4 or IPv6 in a radio interface having a narrow bandwidth. The header compression increases transmission efficiency between radio interfaces by allowing the header part of the data to transmit only the essential information. In addition, the PDCP layer performs a security function in the LTE system. The security function includes ciphering for preventing data wiretapping by a third party, and integrity protection for preventing data manipulation by a third party.
The Radio Resource Control (RRC) layer located at the lowermost portion of the third layer is defined only in the control plane, and controls a logical channel, a transport channel and a physical channel in relation to the configuration, reconfiguration, and release of radio bearers (RBs). In this case, the RBs refer to a logical path provided by the first and second layers of the radio protocol for data transmission between the UE and the UTRAN. In general, configuration (establishment, setup) of the RB refers to the process of stipulating the characteristics of a radio protocol layer and a channel required for providing a particular data service, and setting the respective detailed parameters and operational methods. The RBs include two types: a Signaling RB (SRB) and a Data RB (DRB). The SRB is used as a path for transmitting an RRC message on a C-plane, and the DRB is used as a path for transmitting user data on a U-plane.
Hereinafter, the RLC layer will be explained in more detail. As mentioned above, the RLC layer operates in three modes, TM, UM, and AM. Since the RLC layer performs a simple function in the TM, only the UM and AM will be explained.
The UM RLC generates each Packet Data Unit (PDU) with a PDU header including a Sequence Number (SN), thereby allowing a receiving side to know which PDU has been lost while being transmitted. Accordingly, the UM RLC transmits broadcast/multicast data or transmits real-time packet data such as voice (e.g., VoIP) of a Packet Service domain (PS domain) or streaming on a user plane. Also, on a control plane, the UM RLC transmits, to a specific terminal or specific terminal group in a cell, an RRC message requiring no response for reception acknowledgement.
Similar to the UM RLC, the AM RLC generates each PDU with a PDU header including a Sequence Number (SN). Differently from the UM RLC, in the AM RLC, a receiving side performs acknowledgement for PDUs transmitted from a sending side. In the AM RLC, the reason why the receiving side performs acknowledgement is to request the sending side to retransmit a PDU if the receiving side fails to receive the PDU. The re-transmission function is the main characteristic part of the AM RLC. The AM RLC aims to guarantee error-free data transmission using the re-transmission function. To this end, the AM RLC handles transmission of non-real time packet data such as TCP/IP of PS domain on the user plane, and transmits an RRC message that necessarily requires a reception acknowledgement among RRC message transmitted to a specific terminal in a cell on the control plane.
In terms of directionality, the UM RLC is used for uni-directional communications, while the AM RLC is used for bi-directional communications due to feedback from the receiving side. The UM RLC is different from the AM RLC in the aspect of configuration. The UM RLC and the AM RLC are different in terms of structural aspect: the UM RLC is that a single RLC entity has only one structure of transmission or reception but the AM RLC is that both a sending side and a receiving side exist in a single RLC entity.
The AM RLC is complicated due to its re-transmission function for data. The AM RLC is provided with a retransmission buffer as well as a transmission/reception buffer for retransmission management. The AM RLC performs many functions, e.g., usage of a transmission/reception window for flow control, polling to request a status information (status report) from a receiving side of a peer RLC entity by a sending side, a receiving side's status report informing about its buffer status to a sending side of a peer RLC entity, and generating of a status PDU to transmit status information, or the like. In order to support those functions, the AM RLC requires to have various protocol parameters, status variables, and timers. The PDUs used for controlling data transmission in the AM RLC, such as the status report, a status PDU, or the like, are called Control PDUs, and the PDUs used for transferring user data are called Data PDUs.
In the AM RLC, the RLC Data PDU is further divided into an AMD PDU and an AMD PDU segment. The AMD PDU segment has a portion of data belonging to the AMD PDU. In the LTE system, a maximum size of a data block transmitted by the terminal may vary at each transmission. For instance, having generated and transmitted an AMD PDU having a size of 200 bytes at a certain time period, a sending side AM RLC entity is required to retransmit the AMD PDU since it has received a NACK from a receiving side AM RLC. Here, if a maximum size of a data block which can be actually transmitted is assumed 100 bytes, the AMD PDU cannot be retransmitted in its original form. To solve this problem, the AMD PDU segments are used. The AMD PDU segments refer to the AMD PDU divided into smaller units. During such process, the sending side AM RLC entity divides the AMD DPU into the AMD PDU segments so as to transmit the same over a certain period of time. Then, the receiving side AM RLC entity decodes the AMD PDU from the received AMD PDU segments.
In the related art, the PDCP layer as an upper layer of the RLC has a timer for each PDCP SDU (Service Data Unit). If an ACK is not received until the timer expires, the PDCP layer discards the corresponding PDCP SDU and Protocol Data Unit (PDU), and simultaneously, commands the RLC to discard the corresponding PDCP PDU, i.e., the RLC SDU. Upon receiving the RLC SDU discard indication, the AM RLC would discard the RLC SDU if no segment of the RLC SDU has been mapped to an AMD PDU yet and thusly stored in the RLC buffer. However, if at least one segment of the RLC SDU has already been mapped to an AMD PDU, the AM RLC would not discard the RLC SDU but retransmit the RLC SDU until an ACK is received.
In the related art, there is no restriction on the PDU retransmission by the AM RLC, such as a maximum allowable time or frequency for transmission, or the like. This is because the LTE system having employed technologies, such as OFDM, MIMO, HARQ and the like, assumes that the transmission in the physical layer is stable. That is, since the physical layer has a very low transmission error rate, any restrictions on the retransmission in the RLC layer are not necessary. Therefore, the AM RLC in theory may perform endless retransmission.
For some reasons, however, the RLC may continually fail to retransmit. Among those, a variety of status parameters of the RLC protocol may malfunction by a protocol error (residual error) that is not detected even by the Cyclic Redundancy Check (CRC) in the physical layer, or the receiving side may continually discard the PDUs having successfully been transmitted by the sending side due to different RLC implementation schemes between the terminal and the network. With such RLC-associated problems, there is a possibility to fail even if the retransmission is endlessly performed. Therefore, there is a need to have a solution for the endless retransmission by the RLC.